Peavey 6505+ 112 vs 5150 mk1

I decided to register on this forum, as there seems to be a lot of skilled people on here, who might be able to answer my questions.

I recently bought a Peavey 6505+ 112 combo amp. I bought it used, but only a few months old, and got a great deal.

However, the sound is very disappointing. It does not sound much like a real 5150/6505. I've tried hooking it up to my 4x12, but it still sounds very muddy and undefined, and its voicing is something that would suit classic rock, rather than modern metal. Yeah, I know it's a 6505+, but the plus is said to be tighter, not muddier than the original...

So I looked up the schematic to see how they have changed the circuit from the original. I've gotten through the preamp, focusing only on the lead channel so far. At the end of the tone stack however things get very complicated, and I'm completely lost.

I can tell that the original 5150 has one additional tube between the tone stack and the phase inverter. Is this the reason the 112 sounds like crap?

I'm ready to throw out the reverb and fx loop if needs be...

Attached is the 6505+ schematic. I've noted everything that is correct in red, and what needs to be changed to bring it back to 5150 specs in red.
I've also attached a schematic for the original 5150.

Before we tear into it, what tubes have you tried in it? Both the power tube and preamp tubes. Some tubes are brighter than others. We used to have endless discussions about Clasic 30 tubes. SOme people likes JJ preamp tubes, found them smooth and mellow, others thought the JJs were dull. SOme thought the Sovtek tubes were bright and defines, others found them shrill. Bottom line - the Sovteks were a little brighter than the JJs. After that it was a matter of which sound you prefer. SO you may have some preamp tubes that don;t provide the tone you prefer. Worth spending some time checking.

And the other obvious thing is speaker. But you have tested alternate speakers.

Skipping your main topic a moment, what do you find confusing after the tone stack? The signal leaves the stack, through the master volume, through the channel select relay, then down past the reverb and into the FX loop jacks. (The signal then either goes right through R70 and into the phase inverter, or if you use the loop, through the little driver circuit and the loop jacks) From there into the phase inverter.

The phase inverter itself is very different, the combo uses a split load phase inverter, while the 5150 uses a long tail pair PI. The split load requires only one triode, while the pair needs, well, a pair of them. Your extra tube in the 5150 is a cathode folloower to drive the FX loop, and another triode to recover from it. The combo manages to do without the drive cathode follower by using the small solid state circuit. The recovery triode is still there in the combo.

Circuirt specifics. If you remove R62 as a path to ground, you will have pops every time you switch channels. It is there to keep C14 charged, it is not there INSTEAD of across the other cap. You may wish to explore resistor in parallel with cap there, but leave R62.

Changing C27 from 10nf to 1 nf will roll off less highs and make the amp brighter.

R32 and R46 form a 2/1 voltage divider. Whatever comes through C12 is reduced to roughly a third of its former level. Whatever 47k/(47k + 82K) equals. The 5150 values you have added in are a almost 15/1 reduction, what comes through C12 would be reduced to 1/15 of its former self. They are shifting signal levels at that stage.

Lift R59? Look at your 5150 circuit, the two post controls are wired in parallel, both are 1 meg controls. SO that Ultra Post has a 1 meg in parallel, it happens to be the other pot. WHichever pot is selected, the other one remains in parallel. You would change that. Note in the combo, the separate post controls are in separate channels. The 5150 onbly has one channel, so it switched controls in place. Due to that difference, the combo won;t have the other pot as a parallel, so they added the resistor to take its place. You cxan try it without R59, but not to make it more 5150-like.

R50 (R102?) Well it is 2.2k in one and 2.21k in the other, in other words 2200 ohm and 2210 ohm. If we are worried about that small a difference, half a percent, then I am in the wrong game.

R12, same agrument, I can;t really see the 39k versus 37.4 k thing making much difference.

C5 and C2. They are coupling caps linking three stages together. In the 5150, a 0.002 into the middle stage and a 0.02 coming out. SO at first they limit the bottom end and have a fuller link after the stage. In the combo they make the difference, a 470pf into the stage and a 0.001 leaving. SOunds to me like it would be brighter than the 5150.

Education is what you're left with after you have forgotten what you have learned.

Before we tear into it, what tubes have you tried in it? Both the power tube and preamp tubes. Some tubes are brighter than others. We used to have endless discussions about Clasic 30 tubes. SOme people likes JJ preamp tubes, found them smooth and mellow, others thought the JJs were dull. SOme thought the Sovtek tubes were bright and defines, others found them shrill. Bottom line - the Sovteks were a little brighter than the JJs. After that it was a matter of which sound you prefer. SO you may have some preamp tubes that don;t provide the tone you prefer. Worth spending some time checking.

I must admit I've had my doubts about how much difference the preamp tubes make, and therefore haven't tried replacing the tubes. They're all stock. I recently did a comparison of different tubes in the V1 of my rock master pre, and found the differences to be very subtle. For most brands the differences were too subtle to be noticed:

I don't think a different brand of tubes is enough to make this amp useable, unless of course there is a blown tube somewhere. I'll replace all the preamp tubes and hook up a different power amp to check when I get home in a couple weeks. I've also ordered a bias probe to check the 6l6's and get their bias right.

Originally Posted by Enzo

Skipping your main topic a moment, what do you find confusing after the tone stack? The signal leaves the stack, through the master volume, through the channel select relay, then down past the reverb and into the FX loop jacks. (The signal then either goes right through R70 and into the phase inverter, or if you use the loop, through the little driver circuit and the loop jacks) From there into the phase inverter.

The phase inverter itself is very different, the combo uses a split load phase inverter, while the 5150 uses a long tail pair PI. The split load requires only one triode, while the pair needs, well, a pair of them. Your extra tube in the 5150 is a cathode folloower to drive the FX loop, and another triode to recover from it. The combo manages to do without the drive cathode follower by using the small solid state circuit. The recovery triode is still there in the combo.

My problem is: The circuits are completely different from this point, and no longer just a matter of component values. I don't know a lot about electronics, only just about enough to repair stuff, and to take a piece of one circuit and apply it to another, often with success. I guess a complete rebuild would be neccessary to turn these stages into anything similar to the original design... Would it be any help to bypass the reverb and FX loop alltogether, or would this just be another type of different?

Originally Posted by Enzo

Circuirt specifics. If you remove R62 as a path to ground, you will have pops every time you switch channels. It is there to keep C14 charged, it is not there INSTEAD of across the other cap. You may wish to explore resistor in parallel with cap there, but leave R62.

I see, I'll keep R62 in the circuit.

Originally Posted by Enzo

Changing C27 from 10nf to 1 nf will roll off less highs and make the amp brighter.

This was actually a mistake. 10nf is the correct value here.

Originally Posted by Enzo

R32 and R46 form a 2/1 voltage divider. Whatever comes through C12 is reduced to roughly a third of its former level. Whatever 47k/(47k + 82K) equals. The 5150 values you have added in are a almost 15/1 reduction, what comes through C12 would be reduced to 1/15 of its former self. They are shifting signal levels at that stage.

So, in plain words: these 2 parts define how much signal is passed from the preamp to the tone stack? If so, I guess this has been done to compensate for different gain in the power amp section, and has little to do with tone?

Originally Posted by Enzo

Lift R59? Look at your 5150 circuit, the two post controls are wired in parallel, both are 1 meg controls. SO that Ultra Post has a 1 meg in parallel, it happens to be the other pot. WHichever pot is selected, the other one remains in parallel. You would change that. Note in the combo, the separate post controls are in separate channels. The 5150 onbly has one channel, so it switched controls in place. Due to that difference, the combo won;t have the other pot as a parallel, so they added the resistor to take its place. You cxan try it without R59, but not to make it more 5150-like.

Makes sense!

Originally Posted by Enzo

R50 (R102?) Well it is 2.2k in one and 2.21k in the other, in other words 2200 ohm and 2210 ohm. If we are worried about that small a difference, half a percent, then I am in the wrong game.

I actually misread this value. I thought maybe it said 221k. I wasn't sure, so I put a question mark by this one. 2k2 vs 2k21 is of course negliable.

Originally Posted by Enzo

R12, same agrument, I can;t really see the 39k versus 37.4 k thing making much difference.

In this case maybe they made this change for a reason? Or maybe not... Maybe I'll try to swap it, but like you say, the change probably won't be noticeable

Originally Posted by Enzo

C5 and C2. They are coupling caps linking three stages together. In the 5150, a 0.002 into the middle stage and a 0.02 coming out. SO at first they limit the bottom end and have a fuller link after the stage. In the combo they make the difference, a 470pf into the stage and a 0.001 leaving. SOunds to me like it would be brighter than the 5150.

You mean C5 and C26, right? (C17 and C2 in the 5150 mk1 schematic). I believe these positions are the most important differences between the mk1 and the + preamps, and I should look at these first, after checking that all the tubes are ok.

You seem to be familiar with the 5150 circuit. Do you know the correct voltages are for B+, B++, B+++, Screen, plate etc?

If I typo'd the C numbers, I am sorry, as long as you see what I am geting at.

There is no correct voltage for B+ etc. Seems to me around here, most 5150 amps have something in the 485-500v B+ range. But if your B+ is 480v and your mains are 120v, that means every volt the mains change, the B+ changes 4 volts. SO if your mains are 114v on a hot summer day when all the air conditioners are running for 100 miles in all directions, then another day they are at 125v, like mine here today, that 11 volt mains difference means a range of 44v on the B+. So then what is accurate 495v or 475v? I don;t worry about what the voltage is, I worry about if it looks reasonable. I know the screens will not be much lower than the plates. If my plates are 480 and my screens 250, then something is wrong. If my screen voltage is MISSING, then I know that 400 ohm resistor is probably open. (or 390 when they run out of 400)

Between B+, B++, and B+++ are 10k resistors. SO a change of 1ma through one of those results on a 20v difference from B+ to B+++, 10v per resistor. Individual tubes have different natural currents, so it is hard to say what exactly the voltage would be.

Fender schematics often have a lot of voltage references on them. Darn precise ones in fact. things like 2.46v or whatever. Clearly what they did was take a working amp and measure what they found. The schematics also usually have a note in the corner explaining that voltages are approximate and may vary 20%.

About tubes, each tube position does a different job in this and most every amp. SO a tube that doesn't much matter in V1 might make a wonderful difference in V3. Or wherever. And a tube you just love in V1 might be one you hate in V3, yet the tube you love in V3 might be hated in V1. So to be fair to tubes, you really kinda have to test your selection of tubes in each socket. You might like the EH in V1 and a couple SOvteks elsewhere for example.

A tube change might not be what makes everything wonderful, but it may be just another step in the right direction. After all, which instrument makes a symphony sound wonderful? It takes them all. Flutes may not sound wonderful to you, but the orchestra won;t sound as good without them. (oboes may be a different story)

The bias on this amp is not adjustable without modifying the amp. You will no doubt find the stock bias is much colder than people like to set it. Peavey designs their power amps to be loud and clean, they get the tone from the preamp. remember that 70% thing is just a popular idea, not a requirement.

So, in plain words: these 2 parts define how much signal is passed from the preamp to the tone stack? If so, I guess this has been done to compensate for different gain in the power amp section, and has little to do with tone?

I like to think it is setting the signal level that will feed into the FX loop stages.

39k 37.4k. yeah well 37.4k is 4% off from standard value 39k. Makes a typical 5% 39k part good at that value. 37.4k is a 1% or better value. Some engineer did that because he can. he was probably calculating some target tube current and that was the value he came up with. Rather than shift it to a standar 5% value, he just specified 37.4k. Go ahead and tweak it if you like. I'll bet my lunch money you don;t hear it.

Education is what you're left with after you have forgotten what you have learned.

If I typo'd the C numbers, I am sorry, as long as you see what I am geting at.

no worries, just checking that we're on the same page

Originally Posted by Enzo

There is no correct voltage for B+ etc. Seems to me around here, most 5150 amps have something in the 485-500v B+ range.

ok, I'll check that the voltage is in this range. At this point I'm suspecting that anything could be wrong with this amp. For instance, PV might have installed a cheap out-of-spec input tranny to cut costs. Hence, I want to check that the voltages are within the same ranges as in the original design.

Originally Posted by Enzo

The bias on this amp is not adjustable without modifying the amp. You will no doubt find the stock bias is much colder than people like to set it. Peavey designs their power amps to be loud and clean, they get the tone from the preamp. remember that 70% thing is just a popular idea, not a requirement.

Yep, I'll have to make up a bias mod. I've read that the stock biasing on this amp is about 20mA, which is so low that it will create crossover distortion at all volumes, and reduce headroom a lot. To my understanding, increasing this to 35mA will make the power amp cleaner and more powerful. Maybe Peavey usually design their power amps loud and clean, but in this case I don't think they did...

Originally Posted by Enzo

I like to think it is setting the signal level that will feed into the FX loop stages.

So the +combo has a hotter FX loop than the 5150 head? If I want to bypass the whole fx loop and reverb circuit and to straight from the preamp to the power amp (mainly to rule things out), how do I do this correctly? Do you think this would change the tone audibly?

Yep, I'll have to make up a bias mod. I've read that the stock biasing on this amp is about 20mA, which is so low that it will create crossover distortion at all volumes, and reduce headroom a lot. To my understanding, increasing this to 35mA will make the power amp cleaner and more powerful. Maybe Peavey usually design their power amps loud and clean, but in this case I don't think they did...

O/k, so you have 'read' that 20ma bias will create crossover distortion.
But have you actually observed it on the amp?
I would bet not.
How idling a power tube at 35ma's will make the amp more powerful And cleaner is beyond me.
The higher bias will lower your B+.
So how does that make the amp more powerful.
I would think that a lower bias would.

O/k, so you have 'read' that 20ma bias will create crossover distortion.
But have you actually observed it on the amp?
I would bet not.
How idling a power tube at 35ma's will make the amp more powerful And cleaner is beyond me.
The higher bias will lower your B+.
So how does that make the amp more powerful.
I would think that a lower bias would.

How will a higher bias voltage result in a lower B+? They're not even coming from the same winding on the transformer.

People on forums all around the net says that the sweet spot for 6l6's is somewhere around 35-42mA, but I don't have a clue, and you may be right. I guess I'll have to hook up a sine generator and a scope to see what happens at the output as I adjust the bias voltage.

How will a higher bias voltage result in a lower B+? They're not even coming from the same winding on the transformer.

A little thought will answer that.
The B+ is not a fixed voltage.
It's magnitude is dependant on the current drawn from it.
A higher idle dissipation of the output tubes constitutes a larger draw on the B+.
Which translates to a lower indicated B+ reading.

Besides this, and with due respect: if you have to modify your amp that much, maybe it's better to sell it as is, somebody will love it, and buy the one you want.
Or even build it from scratch, why not?
Problem is that PCB built amps can be easily modded by changing parts values (similar to going blackface from silverface), maybe *pulling* something, and with some effort you can cut a couple traces and reroute with flying wires, but when you have to change the *architecture* of the amp, (which is needed here), you can easily get in the middle of a minefield.
*Best* case, I'd pull the entire PCB and replace it with a new one, if available, but don't know if that's thye case.
jm2c

The B+ and bias may come from different windings, but they still interact. The hotter you set the bias, the more current the tubes draw from the supply, and by extension then the power transformer. The more current you draw from the high voltage winding, the lower its voltage will sag. If nothing else, the wiinding has resistance, and that resistance can be considered in series with the voltage the winding makes. Current through that resistance makes voltage drop - Ohm's Law.

A "higher bias" in Jazz's post meant higher idle current, which is a lower bias voltage. Bias is a voltage, it is not the same as idle current, though we often use the verbal shortcut of referring to the idle current as "the bias."

People on forums all around the net says that the sweet spot for 6l6's is somewhere around 35-42mA, but I don't have a clue, and you may be right. I guess I'll have to hook up a sine generator and a scope to see what happens at the output as I adjust the bias voltage.

Yeah well, if it is on the internet it just MUST be true... SWeet spot is a matter of personal taste. I could put a hotter cam and headers on my mom's car engine, it would be "better." More power, more torque. But will mom see it as better too? You will also find stories of guys who thought their amp was spounding better than it ever had, when their tubes were cherry red and about to melt. SOunded great for about 10 minutes, until the tubes failed. Depends how loud you play. SOme guys play flat out and are LOOKING for power tube distortion. You don;t get power tube distortion without overdriving them, meaning the amp is maxed out. Setting the bias hotter makes it easier to break up the power tubes. But that is only appealing if that is truly your goal. And if you operate at less than full power, none of that happens. Preamp settings don't matter to power tubes.

May I suggest, crossover distortion is a concern of hifi. If the job of the amp was to accurately reproduce some music, then zero Xover is a good goal. But we are not about reproducing anything. The guitar amp is a primary creator of sound, it is PART of the guitar. A little crossover is not usually audible, and even desirable. Think of grilling a steak. A lot of people actually prefer if it is burnt a little. "Get some good char on that." Well, burning the surface of the steak is analog to distortion in an amp. My clumsy point being, set the thing for how it sounds, not for hifi rules.

I agree with Jazz, first check for AUDIBLE and undesirable Xover distortion. Excess distortion and low headroom sounds more like a rationalization than empiricism. PV has made 5150s for MANY years now, and idle currents as low as 11ma to 14ma are common. I hope no one tells me the 5150 lacks headroom. It blows the hair off my head, and that sounds like more headroom...

Running the idle current hotter doesn;t make the amp cleaner, it makes the amp hotter - more ready to break up. People setting their amps hotter arelooking for MORE distortion, not less. The 5150 has no clean channel, the rhythm channel is far from clean. And that brings us back around, it is darn hard to hear small crossover distortions when the preamp is bending the signal into a pretzel.

No, all FX loops operate at about the same level. What each amp has to do to MAKE that level is the difference. If one amp has 2 volts of signal at that point it requires less reduction than one with 32 volts of signal there. The old amp had a hotter signal at that point than the combo, so required greater reduction.

Education is what you're left with after you have forgotten what you have learned.

A "higher bias" in Jazz's post meant higher idle current, which is a lower bias voltage. Bias is a voltage, it is not the same as idle current, though we often use the verbal shortcut of referring to the idle current as "the bias."

Yeh, I thought there had to be some misunderstanding here. Double negatives etc. Jazz was talking voltage, I was talking current.

Originally Posted by Enzo

May I suggest, crossover distortion is a concern of hifi. If the job of the amp was to accurately reproduce some music, then zero Xover is a good goal. But we are not about reproducing anything. The guitar amp is a primary creator of sound, it is PART of the guitar. A little crossover is not usually audible, and even desirable. Think of grilling a steak. A lot of people actually prefer if it is burnt a little. "Get some good char on that." Well, burning the surface of the steak is analog to distortion in an amp. My clumsy point being, set the thing for how it sounds, not for hifi rules.

I agree with Jazz, first check for AUDIBLE and undesirable Xover distortion. Excess distortion and low headroom sounds more like a rationalization than empiricism. PV has made 5150s for MANY years now, and idle currents as low as 11ma to 14ma are common. I hope no one tells me the 5150 lacks headroom. It blows the hair off my head, and that sounds like more headroom...

Running the idle current hotter doesn;t make the amp cleaner, it makes the amp hotter - more ready to break up. People setting their amps hotter are looking for MORE distortion, not less. The 5150 has no clean channel, the rhythm channel is far from clean. And that brings us back around, it is darn hard to hear small crossover distortions when the preamp is bending the signal into a pretzel.

Ahh, that clarifies a lot! higher idle current means less cross-over distortion, but it also means less head room. Got it. But, I'm still curious to hear what happens when I change the bias. Perhaps I want it higher, or maybe I want it lower. I won't know until I've tried.

Originally Posted by Enzo

No, all FX loops operate at about the same level. What each amp has to do to MAKE that level is the difference. If one amp has 2 volts of signal at that point it requires less reduction than one with 32 volts of signal there. The old amp had a hotter signal at that point than the combo, so required greater reduction.

ok, I didn't realize that the 2 capacitor changes early in the preamp circuit would account for such big difference in level at the input of the tone stack.

Originally Posted by J M Fahey

Besides this, and with due respect: if you have to modify your amp that much, maybe it's better to sell it as is, somebody will love it, and buy the one you want.
Or even build it from scratch, why not?
Problem is that PCB built amps can be easily modded by changing parts values (similar to going blackface from silverface), maybe *pulling* something, and with some effort you can cut a couple traces and reroute with flying wires, but when you have to change the *architecture* of the amp, (which is needed here), you can easily get in the middle of a minefield.
*Best* case, I'd pull the entire PCB and replace it with a new one, if available, but don't know if that's thye case.
jm2c

I think you're right. When I first started looking into doing this conversion, I didn't expect the designs to be as different as they are. I'll try a tube swap and a couple simple preamp mods. If that doesn't help, I'll get rid of it.

It's a pity that such a design difference wasn't shown with a *different* model number.
Confusing.

This is in fact very misleading. I thought I was buying a 6505+, and got a very different amp, unsuitable for the purpose I bought it for.
Hopefully there is something wrong with the one I bought. Marketing a mud monster like this as a 6505+ is not fair.

Oh dear lord, how many different amps have worn the name Peavey Bandit?

In their defense, 6505 is to peavey what Chevrolet is to General Motors - a brand. Chevy make everything from the Corvette to the VOlt. Your amp may have an issue or it may just naturally sound this way, but you have traced out the circuit, and other than a few details it is clearly related to the big 6505 amps.

Education is what you're left with after you have forgotten what you have learned.

Oh dear lord, how many different amps have worn the name Peavey Bandit?

In their defense, 6505 is to peavey what Chevrolet is to General Motors - a brand. Chevy make everything from the Corvette to the VOlt. Your amp may have an issue or it may just naturally sound this way, but you have traced out the circuit, and other than a few details it is clearly related to the big 6505 amps.

Well, peavey bandit is different. It's their cheapest line of amps, similar to the Marshall MG, and none of them sound anywhere near impressive, so who cares how the circuit is built?

I like to see it this way: Cheavy makes everything from Corvette to Volt, Peavey make everything from bandit to 6505 (and beyond). If you bought a corvette and the suspension was so loose you had to slow down to 20 every time you wanted to round a corner I bet you'd be disappointed. And that's how I found the 6505+ 112. It looks like a 6505 and has all the horse power (gain) that I expected, but it's too loose and muddy to do most the things you'd want a 6505 for.

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Back to topic:

I changed all the preamp tubes, and it didn't help.

Next I put a patch from the fx loop output and straight to the loop input, and the amp came alive! The whole character of the amp changed, and the noise increased. With the patch cord in, it's still far from perfect, but it's much closer to the sound I expected.

I have no time to look at the circuit right now, but I expect there is something wrong with the fx loop circuit, unless it's just very poorly designed.

Well, peavey bandit is different. It's their cheapest line of amps, similar to the Marshall MG, and none of them sound anywhere near impressive, so who cares how the circuit is built?

I like to see it this way: Cheavy makes everything from Corvette to Volt, Peavey make everything from bandit to 6505 (and beyond). If you bought a corvette and the suspension was so loose you had to slow down to 20 every time you wanted to round a corner I bet you'd be disappointed. And that's how I found the 6505+ 112. It looks like a 6505 and has all the horse power (gain) that I expected, but it's too loose and muddy to do most the things you'd want a 6505 for.

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Back to topic:

I changed all the preamp tubes, and it didn't help.

Next I put a patch from the fx loop output and straight to the loop input, and the amp came alive! The whole character of the amp changed, and the noise increased. With the patch cord in, it's still far from perfect, but it's much closer to the sound I expected.

I have no time to look at the circuit right now, but I expect there is something wrong with the fx loop circuit, unless it's just very poorly designed.

What you basically just did was patch a cable from send to return so sounds like the whole problem is some corroded or cold solder joint jacks so you either need to clean them or replace themor maybe touch them up. Then after that I would look into putting an eq in the loop with a sonic maximizer and a stompbox in front to get the gain and distortion type you are after and you should be good. Of course you could probably get all that tweaking it and adding and eliminating certain components but it would take an awful lot of experimenting and possible trace cutting and hacking which in the long run you'd come out way ahead with what I mentioned.

That's kinda funny. I tend to avoid inserting anything into my 6505+ combo's effects loop as it automatically makes it sound like I'm playing through my rage 158 practice amp I bought about 10 years ago (it gets ridiculously bright and ice-picky)! Probably has something to do with the buffer impedance. I have a feeling without the effects loop engaged, it bypasses the buffer completely and drives straight into the return tube, rolling off high's due to the miller effect. I swapped the tube in this location (the PI/effects return) and it seems to have become *less* noticeable, but noticeable none the less.

To my ears the 6505 and the 6505+ sound very similar. The 6505 has slightly more grind to the tone and is less smooth sounding than the 6505+, but both should be very comparable. Have you tried the head versions of these and tested them? The pre-amp schematic of the 6505+ appears identical to the head version, so a comparison test might be a helpful way to identify potential cold solder/effects loop issues. The power amps are clearly different but it shouldn't make too much of a difference.

That's kinda funny. I tend to avoid inserting anything into my 6505+ combo's effects loop as it automatically makes it sound like I'm playing through my rage 158 practice amp I bought about 10 years ago (it gets ridiculously bright and ice-picky)! Probably has something to do with the buffer impedance. I have a feeling without the effects loop engaged, it bypasses the buffer completely and drives straight into the return tube, rolling off high's due to the miller effect. I swapped the tube in this location (the PI/effects return) and it seems to have become *less* noticeable, but noticeable none the less.

To my ears the 6505 and the 6505+ sound very similar. The 6505 has slightly more grind to the tone and is less smooth sounding than the 6505+, but both should be very comparable. Have you tried the head versions of these and tested them? The pre-amp schematic of the 6505+ appears identical to the head version, so a comparison test might be a helpful way to identify potential cold solder/effects loop issues. The power amps are clearly different but it shouldn't make too much of a difference.

It shouldn't make to much difference if it's working right which obviously his isn't and I put eq's in the loop because it needs them however I agree that other effects don't work well in the loop especially distortion pedals because the loop is -10db down already which is not a guitar level or an amp level. Adding a peak inverter tube change only makes the amp colder and causes a need for a bias change which is why the amp gets ice picky with gain effects or higher gain tubes (or mods to use EL-34's --not recommended) in the loop because it brings it out of crossover distortion which causes this amp to get real loose because it has so much gain and amplitude that hits the grid really hard when biased out of crossover. These amps sound the best with the bias set cold actually into crossover or just barely out of it. EQ's in the loop help dial in that tone and can help get that tinny voicing out and the maximizer tighens the Bass response up. However if you don't have the post control past 3 it will get tinny on you bad and you won't ever get it out until you cross that threshold.